The present invention relates to a method for manufacturing normal as well as ultra-fine cement on the basis of cement materials such as cement clinker, gypsum, slag, limestone, pozzolana, flyash etc. in a plant comprising at least a mill, a classifying separator, one or several separation cyclones and a dust collector, by which method the material is fed to and ground in the mill, and subsequently conveyed to the separator in which it is divided into a coarse fraction which is returned to the mill and into a fine fraction which suspended in a conveying gas is conducted to the cyclone in which the material/gas suspension is separated into cyclone material which is discharged via the material outlet of the cyclone and into dust-laden conveying gas which is discharged via the gas outlet duct of the cyclone, and whereof a partial quantity corresponding at least to the amount of false air present in the installation is subsequently via a duct conducted to and dedusted in the dust collector.
Apparently, there is a strong trend towards incorporation of very fine types of cement, as for example the so-called ultra-fine cement, in the product range of the cement manufacturers. By ultra-fine cement is meant cement having fineness in the range of 600-1000 m.sup.2 /kg, whereas the fineness of normal cement is in the range of 300-500 m.sup.2 /kg.
However, in actual practice it is difficult to manufacture cement having the degree of fineness which characterizes ultra-fine cement. When attempting to manufacture ultra-fine cement by means of open-circuit grinding, the tendency towards dry clogging will often be so pronounced as to result in a substantial deterioration of the grinding economy, and, further, it will often be impossible to manufacture cement having a fineness in excess of 800 m.sup.2 /kg. It is, however, possible to manufacture cement having such a fineness in closed-circuit plants, but this has the disadvantage that the circulating load or the circulation factor across the separator will be quite substantial, which in turn means that the specific power consumption of both the separator and the fan will be very high.
Plants of the kind mentioned in the introduction are known for example from DE 3908752-A1 and DE 3921823-A1. In both specifications methods are described by which cyclone material and dust from the dust collector are intermixed for producing the finished cement, but mention is not made in any of the specifications of utilizing the dust as a basic ingredient for producing ultra-fine cement.